Research in progress serves to define and describe rat and bovine albumin filtration and reabsorption by pinocytosis within the kidney using an unanesthetized rat model. A reproducible fraction of the apical pinocytotic vacuole-lysosomal system is recovered from a 10,000 x G pellet of renal cortex homogenate which contains pinocytosed protein. Bovine albumin enters the system in a predictable manner and comes into equilibrium with the filtered load. A micro solid phase radioimmunoassay is used to measure protein. The system is studied, in the normal state, with "volume expansion proteinuria" and with experimental glomerular disease. Estimates of filtrate protein concentration and bulk flow through the pinocytotic system are possible. The effect of drugs upon the system (vinblastine) are studied. The model is used to study protein catabolism in the proximal tubule as a possible site for autoimmune antigens. In addition, alterations in glomerular permeability with volume expansion can be used to study capillary permeability to protein as a plasma volume regulatory mechanism.